linux-kernel - Re: [PATCH net-next v7 3/4] virtio/vsock: fix logic which reduces credit update messages

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Date:   Mon, 11 Dec 2023 13:01:16 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Arseniy Krasnov <avkrasnov@...utedevices.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Bobby Eshleman <bobby.eshleman@...edance.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...rdevices.ru, oxffffaa@...il.com
Subject: Re: [PATCH net-next v7 3/4] virtio/vsock: fix logic which reduces
 credit update messages

On Thu, Dec 07, 2023 at 01:50:05AM +0300, Arseniy Krasnov wrote:
>
>
>On 07.12.2023 01:08, Michael S. Tsirkin wrote:
>> On Thu, Dec 07, 2023 at 12:52:51AM +0300, Arseniy Krasnov wrote:
>>>
>>>
>>> On 07.12.2023 00:53, Michael S. Tsirkin wrote:
>>>> On Thu, Dec 07, 2023 at 12:18:48AM +0300, Arseniy Krasnov wrote:
>>>>> Add one more condition for sending credit update during dequeue from
>>>>> stream socket: when number of bytes in the rx queue is smaller than
>>>>> SO_RCVLOWAT value of the socket. This is actual for non-default value
>>>>> of SO_RCVLOWAT (e.g. not 1) - idea is to "kick" peer to continue data
>>>>> transmission, because we need at least SO_RCVLOWAT bytes in our rx
>>>>> queue to wake up user for reading data (in corner case it is also
>>>>> possible to stuck both tx and rx sides, this is why 'Fixes' is used).
>>>>> Also handle case when 'fwd_cnt' wraps, while 'last_fwd_cnt' is still
>>>>> not.
>>>>>
>>>>> Fixes: b89d882dc9fc ("vsock/virtio: reduce credit update messages")
>>>>> Signed-off-by: Arseniy Krasnov <avkrasnov@...utedevices.com>
>>>>> ---
>>>>>  Changelog:
>>>>>  v6 -> v7:
>>>>>   * Handle wrap of 'fwd_cnt'.
>>>>>   * Do to send credit update when 'fwd_cnt' == 'last_fwd_cnt'.
>>>>>
>>>>>  net/vmw_vsock/virtio_transport_common.c | 18 +++++++++++++++---
>>>>>  1 file changed, 15 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>>>>> index e137d740804e..39f8660d825d 100644
>>>>> --- a/net/vmw_vsock/virtio_transport_common.c
>>>>> +++ b/net/vmw_vsock/virtio_transport_common.c
>>>>> @@ -558,6 +558,8 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>>>  	struct virtio_vsock_sock *vvs = vsk->trans;
>>>>>  	size_t bytes, total = 0;
>>>>>  	struct sk_buff *skb;
>>>>> +	u32 fwd_cnt_delta;
>>>>> +	bool low_rx_bytes;
>>>>>  	int err = -EFAULT;
>>>>>  	u32 free_space;
>>>>>
>>>>> @@ -601,7 +603,15 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>>>  		}
>>>>>  	}
>>>>>
>>>>> -	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
>>>>> +	/* Handle wrap of 'fwd_cnt'. */
>>>>> +	if (vvs->fwd_cnt < vvs->last_fwd_cnt)
>>>>> +		fwd_cnt_delta = vvs->fwd_cnt + (U32_MAX - vvs->last_fwd_cnt);
>>>>
>>>> Are you sure there's no off by one here? for example if fwd_cnt is 0
>>>> and last_fwd_cnt is 0xfffffffff then apparently delta is 0.
>>>
>>> Seems yes, I need +1 here
>>
>> And then you will get a nop, because assigning U32_MAX + 1 to u32
>> gives you 0. Adding () does nothing to change the result,
>> + and - are commutative.
>
>Ahh, unsigned here, yes.

Ooops, sorry I was confused here!

>
>@Stefano, what did You mean about wrapping here?
>
>I think Michael is right, for example

Yep, I agree!
Sorry for this wrong suggestion!

Stefano

>
>vvs->fwd_cnt wraps and now == 5
>vvs->last_fwd_cnt == 0xffffffff
>
>now delta before this patch will be 6 - correct value
>
>May be I didn't get your idea, so implement it very naive?
>
>Thanks, Arseniy
>
>>
>>
>>>>
>>>>
>>>>> +	else
>>>>> +		fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt;
>>>>
>>>> I actually don't see what is wrong with just
>>>> 	fwd_cnt_delta = vvs->fwd_cnt - vvs->last_fwd_cnt
>>>> 32 bit unsigned math will I think handle wrap around correctly.
>>>>
>>>> And given buf_alloc is also u32 - I don't see where the bug is in
>>>> the original code.
>>>
>>> I think problem is when fwd_cnt wraps, while last_fwd_cnt is not. In this
>>> case fwd_cnt_delta will be too big, so we won't send credit update which
>>> leads to stall for sender
>>>
>>> Thanks, Arseniy
>>
>> Care coming up with an example?
>>
>>
>>>>
>>>>
>>>>> +
>>>>> +	free_space = vvs->buf_alloc - fwd_cnt_delta;
>>>>> +	low_rx_bytes = (vvs->rx_bytes <
>>>>> +			sock_rcvlowat(sk_vsock(vsk), 0, INT_MAX));
>>>>>
>>>>>  	spin_unlock_bh(&vvs->rx_lock);
>>>>>
>>>>> @@ -611,9 +621,11 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
>>>>>  	 * too high causes extra messages. Too low causes transmitter
>>>>>  	 * stalls. As stalls are in theory more expensive than extra
>>>>>  	 * messages, we set the limit to a high value. TODO: experiment
>>>>> -	 * with different values.
>>>>> +	 * with different values. Also send credit update message when
>>>>> +	 * number of bytes in rx queue is not enough to wake up reader.
>>>>>  	 */
>>>>> -	if (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE)
>>>>> +	if (fwd_cnt_delta &&
>>>>> +	    (free_space < VIRTIO_VSOCK_MAX_PKT_BUF_SIZE || low_rx_bytes))
>>>>>  		virtio_transport_send_credit_update(vsk);
>>>>>
>>>>>  	return total;
>>>>> --
>>>>> 2.25.1
>>>>
>>
>